Catch NotUniqueError

[Vayel]

When an index is created on a model, Mongoengine may raise NotUniqueError. Currently, the exception is not caught. It should be and the API should return with the 400 code.

[Vayel]

• https://github.com/umutbozkurt/django-rest-framework-mongoengine/blob/master/rest_framework_mongoengine/serializers.py#L156
• https://github.com/umutbozkurt/django-rest-framework-mongoengine/blob/master/rest_framework_mongoengine/serializers.py#L199